LibGfx: Simplify path storage and tidy up APIs

[MacDue]

Rather than make path segments virtual and refcounted let's store Gfx::Paths as a list of FloatPoints and a separate list of commands.

This reduces the size of paths, for example, a MoveTo goes from 24 bytes to 9 bytes (one point + a single byte command), and removes a layer of indirection when accessing segments. A nice little bonus is transforming a path can now be done by applying the transform to all points in the path (without looking at the commands).

Alongside this there's been a few minor API changes:

• path.segments() has been removed
  • All current uses could be replaced by a new path.is_empty() API
  • There's also now an iterator for looping over Gfx::Path segments
• path.add_path(other_path) has been removed
  • This was a duplicate of path.append_path(other_path)
• path.ensure_subpath(point) has been removed
  • Had one use and is equivalent to an is_empty() check + move_to()
• path.close() and path.close_all_subpaths() assume an implicit moveto 0,0 if there's no moveto at the start of a path (for consistency with path.segmentize_path()).

Only the last point could change behaviour (though in LibWeb/SVGs all paths start with a moveto as per the spec, it's only possible to construct a path without a starting moveto via LibGfx APIs).